1. Field of the Invention
This invention pertains to pipe lowering and lifting apparatus and particularly such apparatus that operates pipe or other tubular goods into and out of a pressurized well.
2. Description of the Prior Art
It is not uncommon in certain fields to drill through formations wherein effective well pressure conditions in excess of 3000 pounds per square inch are encountered, or to repair wells with pressure on the surface control equipment.
In order to operate tubular goods within a well in the presence of high pressure, it has become the practice to pack off, near the surface, the annular space between the casing and the drill pipe or other tubular goods operating within the casing and to install a check valve on the end or inside the drill pipe or other tube operating inside the casing. This check valve is installed to permit passage of fluid downward and prevent upward passage of fluid. In the process of lowering a drill pipe into the well, a continuous length is made up length by length. It may be evident that when the length of pipe in the hole with the check valve installed is such that its weight is less than that of the effective well pressure tending to lift it, the pipe must be forced down until sufficient lengths have been added to the top of the pipe to allow the force of gravity to sink it.
Moreover, when the pipe is retracted from the hole, immediately after the pressure balance point has been reached (that is, the point where the pipe has been shortened by lifting it and unjoining lengths from the top until the weight of the pipe left in the hole just balances the effective force of the well pressure tending to lift it), the well pressure tends to eject the pipe from the casing.
The forceful entry of the pipe until after the balance point is passed by adding sufficient pipe lengths to overcome the lifting force of the well pressure, is termed "snubbing in", and the control of the movement of the pipe under the influence of well pressure in coming out, is termed "snubbing out". The apparatus that is provided to permit control in snubbing in and snubbing out is referred to as snubbing apparatus or merely as the "snubber". It will be realized that with high well pressures, the lifting force exerted on the pipe may become tremendous and if not kept under control could easily result in damage causing great delays and even result in disaster.
To ensure control over a pipe in the above-described environment, it is conventional to employ unidirectional grippers referred to in the industry as "slips". Such slips are similar to a chuck and are used to hold the pipe against movement in either one direction or the other. When two sets of slips are placed back-to-back, they then hold the pipe against movement in either direction. It is further conventional to employ such sets or pairs of slips at two locations: (1) on a vertical traveling table or support (herein referred to as the traveling support) and (2) on a stationary platform or support. Hence, it may be seen, that during a snubbing operation the slips on the stationary support are released while the slips on the vertical traveling support are in their gripping state. Once the stroke is complete, then the slips on the stationary support are actuated to grip the pipe and the slips on the traveling support are released to allow for repositioning of the traveling support with respect to the pipe for another stroke.
In normal operation in the absence of pressure (or when the pipe is of sufficient length that operation is below the pressure balance point), there is no need to be able to apply downward pressure in addition to that applied by gravity to the pipe. The traveling support is suspended on the cable usually provided on a conventional drilling rig or rig used in well repair or servicing. The cable is reeled onto or off of a conventional winch drum complete with brake and clutches to enable the operator to lower or hoist the traveling support. This conventional winch with its accessories is commonly called the "draw works". The draw works provides adequate pipe hoisting and lowering capability when well pressure is not a factor. When well pressure is a factor, external means is applied to the pipe to push the pipe into the well. This pushing down is usually done by sophisticated machines under control of others than the rig operator.
For example, one common method employed to provide downward pressure is to provide a plug in the pipe, normally in the form of a check valve, and pump drilling fluid through the pipe. Pump pressure in excess of well pressure causes pumped fluid to enter the pipe and travel downward through the check valve. This method leaves the pipe at atmospheric pressure above the check valve and at well pressure below the check valve. Hence, succeeding lengths of pipe can be screwed together and forced into the well without loss of pressure through the bore of the pipe.
In the more sophisticated rigs, hydraulic, pneumatic or electrical driving means is provided for applying both upward as well as downward force to the pipe. In some hydraulic rigs, accumulators are used to provide energy storage of the hydraulic fluid during hydraulic piston retraction. This provides an energy saving that would not otherwise be available. Such rigs, although more automated than the more simple rigs described above, are usually much heavier and require more time to set up and take down. Further, in the event of malfunction, repairs are more time consuming and costly, causing expensive rig downtime, as well.
Further, control over the applied hydraulic pressure is via gauges observable to the operator, but not to others.
Therefore, it is a feature of the present invention to provide an improved snubbing apparatus that allows the rig operator to have control over applying positive pressure during both "snubbing in" and "snubbing out" operations.
It is still another feature of the present invention to provide an improved snubbing apparatus using a push-pull cable arrangement on the traveling support, the cables kept in constant tension, to provide visual means to the rig workers that operation of the traveling support is satisfactory.
It is yet another feature of the present invention to provide an improved snubbing apparatus for applying both downward and upward pressure and using hydraulic pistons cable-connected to the underneath side of the traveling support that work in conjunction with the cable from which the traveling support is suspended.
It is still another feature of the present invention to provide an improved snubbing apparatus which maintains a preset tension in operating cables, at least one cable being connected to a hydraulic piston, the piston, in turn, being connected to an accumulator to provide energy conservation to rig operation.
It is yet another feature of the present invention to provide an improved snubbing apparatus suitable for adapting to almost any conventional rig and providing positive control to the rig operator in the form of light-weight, highly efficient and easy to set up equipment.